Effect of Tonic Pain on Motor Acquisition and Retention while Learning to Reach in a Force Field

Abstract

Most patients receiving intensive rehabilitation to improve their upper limb function experience pain. Despite this, the impact of pain on the ability to learn a specific motor task is still unknown. The aim of this study was to determine whether the presence of experimental tonic pain interferes with the acquisition and retention stages of motor learning associated with training in a reaching task. Twenty-nine healthy subjects were randomized to either a Control or Pain Group (receiving topical capsaicin cream on the upper arm during training on Day 1). On two consecutive days, subjects made ballistic movements towards two targets (NEAR/FAR) using a robotized exoskeleton. On Day 1, the task was performed without (baseline) and with a force field (adaptation). The adaptation task was repeated on Day 2. Task performance was assessed using index distance from the target at the end of the reaching movement. Motor planning was assessed using initial angle of deviation of index trajectory from a straight line to the target. Results show that tonic pain did not affect baseline reaching. Both groups improved task performance across time (p<0.001), but the Pain group showed a larger final error (under-compensation) than the Control group for the FAR target (p = 0.030) during both acquisition and retention. Moreover, a Group x Time interaction (p = 0.028) was observed on initial angle of deviation, suggesting that subjects with Pain made larger adjustments in the feedforward component of the movement over time. Interestingly, behaviour of the Pain group was very stable from the end of Day 1 (with pain) to the beginning of Day 2 (pain-free), indicating that the differences observed could not solely be explained by the impact of pain on immediate performance. This suggests that if people learn to move differently in the presence of pain, they might maintain this altered strategy over time.

Pain Induced during Both the Acquisition and Retention Phases of Locomotor Adaptation Does Not Interfere with Improvements in Motor Performance

“…The fact that participants of each group adopted different motor strategies during the locomotor adaptation emphasises the fact that motor skills can be acquired differently with pain although the task goal may still be reached. Previous studies have shown reorganisation of muscle activity with pain or alterations in kinematics while maintaining the global motor performance intact in line with changes in motor strategy observed in the present study [9, 24, 25]. …”

“…In the central nervous system (CNS), sensory information available during motor practice would be associated with the goal of the task and the state of the motor system to form a representation of the motor skill, which would contribute to the specificity of practice effect [14, 15]. Another aspect that might impact the ability to transfer a motor skill from a “pain context” to a “pain-free context” is the fact that pain has been reported to influence motor strategies used during motor adaptation tasks, even when the global performance itself is not affected [9]. …”

Optimal control of reaching is disturbed in complex regional pain syndrome: a single-case study

“…Additionally, pain stimulants have been found to interfere with FF control in preparation for movement. This minimizes trajectory corrections in the FB control period 11. Considering that we observed impaired motor control during the FF and FB periods in our participant, pathological pain might first disrupt precise goal-directed movements during the FB control period.…”

“…Movement disorders such as Parkinson’s disease and cerebellar ataxia disturb goal-directed reaching movement 8,9. Acute experimental pain in healthy subjects has also been found to disturb goal-directed reaching movement 10,11. However, few studies have investigated reaching control in patients with chronic pain in an upper limb.…”

Virtual feedback for motor and pain rehabilitation after spinal cord injury

“…8 In addition, pain has been shown to directly interfere with sensorimotor functions, such as the capacity to learn locomotor 9 and upper limb motor 10 tasks. As such, simultaneous treatment of motor functions and pain would be preferred over conventional treatment, that is suboptimal as evidenced by the high prevalence of chronic pain.…”